Ice cream

At a particular ice cream van two types of ice creams are being sold. The small one costs 1 dollar, the big one 2 dollars. Marsha puts an amount of 2 dollars one the counter and gets without asking the big ice cream. Martha also would like to have an ice cream. She also puts an amount of 2 dollars on the counter. Then the ice cream seller asks her which ice cream she would like to have. Assuming that the persons do not know each other, how can you explain their behavior.

Gas, water and electricity

In a suburb three new houses all have to be connected to a gas (red), water (blue) and electricity (green) supply. All houses should get their own gas, water and electricity connection. Furthermore no connections may cross each other. Because of the danger of stealing, no connections are laid below a house. In the figure on the right-hand side the situation is sketched. The red squares are the houses, the circles the point from which the connections should go to the houses. Why is it impossible to lay down all connections in such a way that no connection crosses another connection?

Horse inheritance

The last will of a horse breeder is that all his horses are to be given to his three children in the following ratio: 1/2:1/3:1/9. At the moment the horse breeder passed away he owned 17 horses. How could his last will be followed?